<html>
<head></head>
<body>
<iframe id="myiframe" width="400" height="300" src="http://vinaphone.com.vn/login.jsp?lang=vi"></iframe>
<button id="btnTest">Fill the form</button>
  <script>    
     function load() {
        var btn = document.getElementById('btnTest');
        btn.onclick = function(){
        var ifrm = document.getElementById('myiframe');
        ifrm.contentWindow.document.forms['smsFormOnline'].bSubs.value = 'Hello world!';
		//ifrm.document.getElementById('username').value = "Something";
     };    
   }
   window.onload = load;
  </script>
</body>
</html>